The decision to activate an immune response is made by unique antigen presenting cells, called dendritic cells that are resting until they encounter foreign microorganisms or inflammatory stimuli. Early activated dendritic cells trigger innate immune responses that represent the first line of defense against invading pathogens to limit the infections. At later times after activation they prime antigen-specific immune responses to clear infectious pathogens and give rise to immunological memory. Dendritic cells control the responses of several types of lymphocytes, and are key in the transition between innate and adaptive immunity. They determine the class of the T cell response, including the development of Th1 cells to eradicate pathogens, or conversely unresponsiveness or tolerance, and thus are critical for the development of an effective immune response with minimum immunopathology. [unreadable] [unreadable]